Superconductors are materials, which exhibit no measurable resistance below a critical temperature Tc. Superconductor materials also have a critical current 1c, above which the material is normally conductive and below which the material is superconductive. Ic depends on the temperature and the external magnetic field.
Materials with a critical temperature>20 K are described as high-temperature superconductors (HTS). Because of their thin construction 2nd generation high-temperature superconductors (2G HTS) are particularly suitable for this insulation. These have superconductive layers with a thickness of 1-10 μm, preferably of yttrium barium copper oxide (YBCO), which are applied to a 10-200 μm thick, electrically conductive substrate, in particular of a nickel chrome molybdenum alloy known as Hastelloy C276, or of a nickel tungsten alloy. The company SuperPower Inc. shows the construction of a 2G HTS wire on its website, <<http://www.superpowerinc.com/content/2 g-hts-wire>>.
In the superconductive components of resistive current limiters, the arrangement of the 2nd generation high-temperature wire superconductor is typically flat bifilar pancake spirals, as presented by W. Schmidt and H.-P. Kramer in the 4th Braunschweig Superconductivity Seminar, 2009; see page 12 of the presentation at <<http://www.tu-braunschweig.de/Medien-DB/iot/8-supraleitende-strombegrenzer-aus-ybco-bandleitern-h-p-kraemer.pdf>>.
When designing the insulation of these bifilar spirals the following physical effects are to be observed:                When the current flows through twists of the spirals Lorentz forces act, having a symmetrising effect in the radial direction. In the axial direction, unavoidable minimal asymmetries in the construction of the spirals, however, have a destabilising effect (force acting on adjacent conductors in the opposite axial direction), the following being the case: the greater the asymmetry the greater the force.        When alternating frequency is used, the conductor vibrates at double the frequency, i.e. if the applied supply frequency is 50 Hz then 100 Hz vibrations occur in the conductor.        When quenched in a nitrogen bath, additional gas formation and pressure build-up act on the conductor. When liquid nitrogen vaporises, about 700 times the volume of gas is created, i.e. the liquid that has penetrated below the insulation can inflate and damage the insulation during quenching.        
In bifilar spirals, high-voltage insulation with a useful thickness adjusted to the voltage values is required. Known insulation methods for high-temperature superconductors are wrapping, varnishing and co-extrusion, which have overall problems with the sharp edges of the 2G HTS however. Because of the 100 Hz vibrations referred to, sharp edges bring about the risk of abrasion with damage to the thin insulating layer in the region of the edge.
Wrapping is only possible with very thin layers of insulation because of the sharp bend at the edge of a wire. Overlapping is unavoidable and as a result a conductor of irregular thickness is maintained. This is shown by way of example on the datasheet Amperium Wire Insulation (September 2010) by the company American Superconductor, which is available on the Internet at <<http://www.amsc.com/products/htswire/InsulatedWire.html>>.
Insulation by means of varnishing is also difficult to produce because, for example, in the case of a 100 μm thick conductor a 100 μm thick layer of varnish cannot be achieved.
Co-extrusion for high-temperature superconductors is known from WO 01/61712 A1 and WO 03/073439 A1.